An apologetic Boston Celtics forward Jared Sullinger is free on $5,000 bail after he was arrested yesterday on charges of assaulting his longtime girlfriend last weekend after she “found evidence on his cellphone that he was cheating on her,” Waltham cops said.

Sullinger pleaded not guilty in Waltham District Court yesterday to assault and battery, destruction of property of more than $250 and witness intimidation.

“The experience was humbling and embarrassing for me,” Sullinger said in a statement. “Yet I know that this situation has brought both sorrow and embarrassment to my girlfriend, my family, the Boston Celtics organization, my teammates and my fans. To all of you, I apologize from the bottom of my heart.”

The Boston Celtics’ Jared Sullinger stood stone-faced and wearing handcuffs in the dock of Waltham District Court on Tuesday as prosecutors said the 6-foot-9-inch, 260-pound power forward pinned his girlfriend to a bed and to the floor of their townhouse last weekend and then snatched and smashed her cellphone when she tried to call police.

“During this heated argument in the bedroom, Jared pushed her down onto the bed and got on top of her,” Waltham police wrote in their report of the confrontation. Sullinger’s girlfriend said “she tried several times to get up, but he kept pinning her down and would not let her up.’’